The present invention relates generally to sewing machines of the type wherein a thread-carrying needle is laterally shifted to form sewn stitches in a zig-zag pattern and, more particularly, relates to an arrangement for optimally positioning a hook of a looptaker of the sewing machine relative to laterally shifted loop-forming positions of the needle.
Sewing machines capable of forming an ornamental chain of lock stitches in laterally-shifting, zig-zag pattern are well known and in widespread commercial use. In a typical commercial zig-zag sewing machine, the thread-carrying needle is driven from a main drive shaft of the machine through intermediary eccentric cam mechanisms to reciprocate vertically upwardly and downwardly through a throat plate in a stitching bed of the machine frame while the needle is laterally shifted leftwardly and rightwardly in alteration in timed relation to the vertical reciprocatory movements of the needle. A looptaker having a peripheral hooked beak portion, commonly referred to as a hook, is rotatably driven beneath the throat plate in parallel relation to the laterally shifting movements of the needle by a secondary drive shaft driven from the main drive shaft in timed relation to the needle reciprocating vertical and laterally shifting movements. A thread-carrying bobbin is mounted stationarily alongside the looptaker. In operation, upon the completion of each downward stroke of the needle, a loop of the thread carried by the needle is formed as the needle begins its upward stroke, the timing of the rotation of the looptaker in relation to the needle being such that the hook of the looptaker seizes the loop and carries it around the bobbin to lock stitch the threads of the needle and bobbin together.
A long-standing and widely recognized problem in the operation of zig-zag sewing machines of the aforementioned type is that, since the looptaker is conventionally rotated at a constant angular speed about a fixed axis, the looptaker cannot present the hook in an optimal disposition with respect to the needle at both the laterally shifted leftward and rightward positions of the needle. Accordingly, it is conventional practice to coordinate the rotation of the looptaker with respect to the needle reciprocation as though the needle were being reciprocated in a non-shifting straight stitch position equidistant the leftward and rightward shifted needle positions for the zig-zag stitch. In this manner, the hook is equally out of optimal timed relationship with the needle at each of the needle's leftwardly and rightwardly shifted positions, whereby the hook prematurely takes the thread loop when the needle is shifted rightwardly in the direction towards the approaching hook and likewise is delayed in taking the loop when the needle is shifted leftwardly in the opposite direction away from the approaching hook. While the sewing machine is "acceptably" operable in this manner, missed stitches, broken needles, and prematurely worn hooks do occur.
Various proposals have been made to regulate the depth of needle penetration through the throat plate with respect to the hook to compensate for the shifted positions of the needle to improve the timing of the needle with respect to the hook, as representatively disclosed in U.S. Pat. Nos. 1,159,523; 2,932,268; and 3,779,187. However, none of these arrangements are known to have met with any significant degree of commercial acceptance and success. In other types of zig-zag sewing machines, the looptaker or other looper device is arranged for lateral shifting in timed relation to the lateral shifting of the needle to achieve proper relative timing, as representatively disclosed in U.S. Pat. Nos. 2,690,723; 3,490,401; and 3,783,810. It is also known in another type of zig-zag sewing machine to provide a cam-controlled mechanism for intentionally producing a pattern of missed stitches by selectively advancing or retarding rotation of the looptaker out of timed relationship with respect to the needle, as disclosed in U.S. Pat. No. 3,804,042.
Finally, it is also known to angularly accelerate and decelerate the looptaker in timed relation to the shifting movements of the needle to position the hook of the looptaker in optimal relationship to the needle at each shifted position thereof, as disclosed in U.S. Pat. No. 4,924,788. While this latter technique is considered superior to the aforementioned proposals, there still remains a potential for further improvement over the angular acceleration and deceleration arrangements disclosed in this particular patent.